I am attempting to get the information of user whenever user logged in to the website, it success when I used a DataSet, but if I want to use the SqlDataReader, the error says: Invalid attempt to read when reader is closed. I have search why is it like that and I have found an article says that
SqlDataReader requires connection remains open in order to get the
data from the server, while DataSet does not need requires
connection remains open.
My question is: I want to know how can I use SqlDataReader as well? So that I don't have to depends on DataSet all the times when I want to get the data from the database.
My problem is occurs when I am trying to change the structure of reading the data function using SqlDataReader, so that it can be re-usable anytime.
Here is the code:
DatabaseManager class:
public SqlDataReader GetInformationDataReader(string procName, SqlParameter[] parameters)
{
SqlDataReader reader = null;
using (SqlConnection conn = new SqlConnection(connectionString))
{
conn.Open();
using (SqlCommand cmd = new SqlCommand(procName, conn))
{
cmd.CommandType = CommandType.StoredProcedure;
if (parameters != null)
{
foreach(SqlParameter parameter in parameters)
{
cmd.Parameters.Add(parameter);
}
}
reader = cmd.ExecuteReader();
}
}
return reader;
}
Web Manager class:
public ModelContexts.InformationContext GetInformation(string username)
{
SqlDataReader reader = null;
ModelContexts.InformationContext context = new ModelContexts.InformationContext();
SqlParameter[] parameters =
{
new SqlParameter("#Username", SqlDbType.NVarChar, 50)
};
parameters[0].Value = username;
try
{
reader = DatabaseManager.Instance.GetInformationDataReader("GetInformation", parameters);
while(reader.Read())
{
context.FirstName = reader["FirstName"].ToString();
context.LastName = reader["LastName"].ToString();
context.Email = reader["Email"].ToString();
}
}
catch(Exception ex)
{
throw new ArgumentException(ex.Message);
}
return context;
}
Controller:
public ActionResult MainMenu(ModelContexts.InformationContext context, string firstName, string lastName, string username, string email)
{
context = WebManager.Instance.GetInformation(User.Identity.Name);
firstName = context.FirstName;
lastName = context.LastName;
username = User.Identity.Name;
email = context.Email;
return View(context);
}
Model contains string return value with getter and setter (FirstName, LastName and Email).
View contains the html label and encode for FirstName, LastName and Email from the Model.
Appreciate your answer.
Thanks.
Here is an approach you can use to keep the code pretty clean that allows you to read from the SqlDataReader while the connection is still open. It takes advantage of passing delegates. Hopefully the code is understandable. You can adjust it to fit your specific needs, but hopefully it illustrates another option at your disposal.
public void GetInformationDataReader(string procName, SqlParameter[] parameters, Action<SqlDataReader> processRow)
{
SqlDataReader reader = null;
using (SqlConnection conn = new SqlConnection(connectionString))
{
conn.Open();
using (SqlCommand cmd = new SqlCommand(procName, conn))
{
cmd.CommandType = CommandType.StoredProcedure;
if (parameters != null)
{
foreach(SqlParameter parameter in parameters)
{
cmd.Parameters.Add(parameter);
}
}
using (SqlDataReader dataReader = cmd.ExecuteReader())
{
while (dataReader.Read())
{
// call delegate here.
processRow(dataReader);
}
}
}
}
return reader;
}
public ModelContexts.InformationContext GetInformation(string username)
{
SqlDataReader reader = null;
ModelContexts.InformationContext context = new ModelContexts.InformationContext();
SqlParameter[] parameters =
{
new SqlParameter("#Username", SqlDbType.NVarChar, 50)
};
parameters[0].Value = username;
try
{
// Instead of returning a reader, pass in a delegate that will perform the work
// on the data reader at the right time, and while the connection is still open.
DatabaseManager.Instance.GetInformationDataReader(
"GetInformation",
parameters,
reader => {
context.FirstName = reader["FirstName"].ToString();
context.LastName = reader["LastName"].ToString();
context.Email = reader["Email"].ToString();
});
}
catch(Exception ex)
{
throw new ArgumentException(ex.Message);
}
return context;
}
Brief explanation:
You'll notice that the overall structure of the code is very similar to what you already have. The only changes are:
Instead of returning a SqlDataReader, the GetInformationDataReader() method accepts an Action<SqlDataReader> delegate.
Within the GetInformationDataReader() method, the delegate is invoked at the correct time, while the connection is still open.
The call to GetInformationDataReader() is modified to pass in a block of code as a delegate.
This sort of pattern can be useful for exactly these cases. It makes the code reusable, it keeps it pretty clean and separate, and it doesn't prevent you from benefiting from the using construct to avoid resource/connection leaks.
You have wrapped your SqlConnection object in a using clause, therefore at the end of it SqlConnect.Dispose is called, closing the connection. Whatever caller is consuming your SqlDataReader no longer has the open connection, therefore you're getting your error.
while DataSet does not need requires connection remains open.
That is not entirely correct. DataSet is just an object that is typically filled when called by SqlDataAdapter (the Fill() method of that class). The SqlDataAdapter handles the opening and closing of the SqlConnection, which is most likely why that comment states that. But it's a different class that handles that, not the DataSet itself. Think of the DataSet as just the object that holds the result set of the SqlCommand.
To answer your comment...
So, shouldn't I use using keyword for this matter? In all of the Sql keyword?
I wouldn't take that approach either. You could have a connection leak bug quite easily with that model, and running out of pooled connections could be a not-so-fun thing to troubleshoot.
Typically it's best to consume your data and then close/dispose your connection. There's a saying, "open late, close early". That's typically how you'd want to approach this. I wouldn't try to pass a SqlDataReader object between class methods for this very issue that you're dealing with. The workaround (leaving the connection open) is very error prone.
Another though process, going back to something we mentioned, don't use the SqlDataReader. You have no benefit to cyclically loop through reading each row. Depending on your result set, just fill a DataSet (or usually more appropriate, a DataTable) and return either that Data[Set | Table] or, even better, an object that is more representative of the data it pertains to.
Related
I am working on a project in VS13 and SQL Server 2012 and I am facing a little problem. I can't seem to be read data from my database. It throws exception whenever I try to do any operation on the SqlDataReader object I use to read the data.
The exception I am getting is InvalidOperationException.
Look at this code of mine, I call this function with the SQL query as the parameter and stores the returned object in another SqlDataReader object.
private SqlDataReader reader (string sqCommand)
{
myConnection.Open();
string string1;
string1 = sqCommand;
SqlDataReader a = null;
try
{
SqlCommand Newcommand = new SqlCommand(string1, myConnection);
a = Newcommand.ExecuteReader();
}
catch (Exception e)
{
MessageBox.Show(e.ToString());
}
myConnection.Close();
if (a.Read()) //**statement 1**
return a;
else
return null;
}
I gets the exception in the statement 1 marked in the code above and in every operation I perform on the object a or the object that receives this.
Can anyone please tell me what is wrong with my code or provide any other explanation? If more other parts of the code is needed for the purpose of finding for the error, I can provide it.
Thanks for the time and the help you may provide. :)
Your connection needs to stay open while you're using the SqlDataReader. A more conventional use for your SqlDataReader would be as follows:
private List<object> Reader(string sqCommand)
{
using (SqlConnection myConnection = new SqlConnection(ConnectionString))
{
myConnection.Open();
using (SqlCommand cmd = new SqlCommand(sqCommand, myConnection))
using (SqlDataReader reader = cmd.ExecuteReader())
{
List<object> list = new List<object>();
while (reader.Read())
{
list.Add(reader[0]);
}
return list;
}
}
}
I've written a function to perform MySQL statements. In this function I give in a statement and get back the MySqlDataReader, but the problem is my function do not close the connection. After a short while of using the Programm, it crashs because the new connection can't be open. This is the error i got by trying open the new connection:
error connecting: Timeout expired. The timeout period elapsed prior
to obtaining a connection from the pool. This may have occurred
because all pooled connections were in use and max pool size was
reached.
My code look like this:
MySQL Class:
class mySql
{
string cs = "server=123.123.123.123;" +
"uid=abcabc;" +
"pwd=123456;" +
"database=overflow_test;";
private MySqlConnection conn_f() // create a Connection
{
MySql.Data.MySqlClient.MySqlConnection conn;
conn = new MySql.Data.MySqlClient.MySqlConnection();
conn.ConnectionString = cs;
try
{
conn.Open();
return conn;
}
catch
{
return null;
}
}
public MySqlDataReader CMD_f(string comand) //execute SQL Command
{
MySql.Data.MySqlClient.MySqlCommand cmd;
cmd = new MySql.Data.MySqlClient.MySqlCommand();
MySqlConnection conn = conn_f();
cmd.Connection = conn;
cmd.CommandText = comand;
cmd.Prepare();
rdr = cmd.ExecuteReader();
return rdr;
}
}
and an example how i use it Main Class
class main{
mySql DB = new mySql();
public void main(){
MySqlDataReader rdr = DB.CMD_f("SELECT * FROM tbl_kategorie");
int i = 0;
while (rdr.Read())
{
string str = rdr.GetString(1);
Console.WriteLine(str);
}
}
Has someone an Idea to solve the Problem.
Sincere regards LFS96 (Fabian Harmsen)
The main problem is the inbalance in the code. The CMD_f method creates a connection, but doesn't take responsibility for it. You should rather make the method that creates the connection public, so that you can take responsibility for it in the code that can close it.
That's a bigger change, so first let's look at a smaller change to fix the code.
The data reader should expose the data connection as the Connection property. I don't see that in the documentation, so it's possible that it doesn't, but the data readers in the .NET framework does.
If the property is exposed, then you can make a quick fix to make the code work with minimal change:
MySqlDataReader rdr = DB.CMD_f("SELECT * FROM tbl_kategorie");
int i = 0;
while (rdr.Read())
{
string str = rdr.GetString(1);
Console.WriteLine(str);
}
rdr.Close();
rdr.Connection.Close();
If you make the conn_f method public and don't call it in the CMD_f method, you can write much more robust code, that never leaves a connection or data reader hanging even if there is an error. That's a bigger change in the code, but definitely worth the effort when you have time to implement and test it. Using a using block to make sure that the objects are always disposed correctly makes the code much more resilient:
using (MySqlConnection conn = DB.conn_f()) {
using (MySqlDataReader rdr = DB.CMD_f(conn, "SELECT * FROM tbl_kategorie")) {
int i = 0;
while (rdr.Read()) {
string str = rdr.GetString(1);
Console.WriteLine(str);
}
}
}
Side note: Identifiers in C# tend to be descriptive. I suggest names like DB.CreateConnection and DB.ExecuteReader rather than DB.conn_f and DB.CMD_f.
I'm rewriting part of an old webforms application. I want to make a central function that will do the select queries. I will feed it the SQL query and parameters and it will do the rest.
So far I have this:
MySqlDataReader DoRead(string query, params MySqlParameter[] pms)
{
MySqlCommand myCommand;
if (!myConnection)
myConnection = new MySqlConnection(sCon);
if (myConnection.State != ConnectionState.Open)
{
myConnection.Close();
myConnection.Open();
}
myCommand = myConnection.CreateCommand();
myCommand.CommandText = query;
foreach (MySqlParameter p in pms)
{
myCommand.Parameters.Add(p);
}
return myReader = myCommand.ExecuteReader();
}
My question is, can I save the results of myReader to a variable and return that same variable instead of myReader just so I could close the connection and the reader immediately in the function instead in the main code?
Of course. And that's exactly what you should be doing, for exactly the reason you state:
so I could close the connection and the reader immediately in the function
The problem you're facing is the use of the reader in the first place, which itself is coupled to the open data stream. So if consuming code is expecting a reader, it's going to have to change in order to fix this.
How does the application later use this reader? If you're trying to make a generic function, then the result needs to be pretty generic too. So if consuming code is all doing custom things with the reader then perhaps you can instead return a DataSet and consuming code can do custom things with that instead. Something like this:
var result = new DataSet();
using(var myConnection = new MySqlConnection(sCon))
{
myConnection.Open();
var myCommand = myConnection.CreateCommand();
myCommand.CommandText = query;
foreach (var p in pms)
myCommand.Parameters.Add(p);
var myAdapter = new MySqlDataAdapter(myCommand);
myAdapter.Fill(result);
}
return result;
(Note that I made another change here as well. The connection object should also be local to the scope of the method. Shared connection objects open up a world of potential problems, one of which you've undoubtedly tried to fix with that conditional to close/open the connection state. Just avoid that world of problems entirely and dispose of connections once you're done with them.)
I have 2 methods as below :
internal static SqlDataReader SelectData(string sql)
{
using (var sqlConnection = new SqlConnection(Constant.ConnectionString))
{
sqlConnection.Open();
var sqlCommand = new SqlCommand(sql, sqlConnection);
var dataReader = sqlCommand.ExecuteReader();
return dataReader;
}
}
============
And using this method as :
var dataReader = SelectData(---some sql ---);
private void AddData(dataReader)
{
while (dataReader.Read())
{
Employee e = new Employee();
e.FirstNamei = dataReader["Name"].ToString();
}
dataReader.Close();
}
I know we can merge this two method, but I am looking at better way write this, OR this can cause some problem??
Actually you are in fact leaving yourself open a bit. You really want to write it like this:
using (SqlConnection cnn = new SqlConnection(cnnString))
using (SqlCommand cmd = new SqlCommand(sql, cnn))
{
// use parameters in your SQL statement too, so you can do this
// and protect yourself from SQL injection, so for example
// SELECT * FROM table WHERE field1 = #parm1
cmd.Parameters.AddWithValue("#parm1", val1);
cnn.Open();
using (SqlDataReader r = cmd.ExecuteReader())
{
}
}
because you need to make sure these objects get disposed. Further, by going this direction you don't need dataReader.Close(). It will get called when it gets automatically disposed by the using statement.
Now, wrap that collection of statements inside a try...catch and you're in business.
A couple of things
1) Since you're closing your connection on SelectData, dataReader should blow up on AddData as it requires an open connection
2) AddData shouldn't close dataReader as he didn't open it.
3) Maybe you're hiding some code but I don't see that you use Employee instance created on AddData
Technically, first method would be correct if you would do
sqlCommand.ExecuteReader(CommandBehavior.CloseConnection);
Then, your client would close the reader and your connection will be closed as well.
Second example would also be correct if you didn't close the reader inside of it. There is no criminal in passing the reader to a method just to iterate it. but it has to be controlled from where it was created. How you open and dispose of it - this is different question.
i work with N tiers tec in C# for ado, trying to make it easy to use and capable to change any database kind with out write all the cod all over again ,
my code here doesn't get any error but it doesn't get any values to my textbox
(i am trying to get data from table to many textboxs to update it later)
and here how code works:{
at first i make some functions to take any set any kind of parameters or set any command and then i make other function to to execute what ever i set or get from database all that Function i build it in folder name (Data Access Layer)
then i made other folder (Data Build layer)to take use all those function for what ever i want to do in any page (insert , update , delete , Select),
the last think i do it to call the function i made at at (Data Build layer) to my page or control ,
i do all that because if i Change the database Type ,i change only one class and other classes still the same
i hope i explain enough (sorry for my English not good enough)}
Code :
Class DataAccessLayer
public static void Setcommand (SqlCommand cmd,CommandType type,string commandtext)
{
cmd.CommandType=type;
cmd.CommandText=commandtext;
}
public static void AddSQLparameter(SqlCommand cmd, int size,SqlDbType type,object value,string paramName,ParameterDirection direction)
{
if (cmd == null)
{
throw (new ArgumentException("cmd"));
}
if (paramName == null)
{
throw (new ArgumentException("paramName"));
}
SqlParameter param=new SqlParameter();
param.ParameterName= paramName;
param.SqlDbType=type;
param.Size=size;
param.Value=value;
param.Direction=direction;
cmd.Parameters.Add(param);
}
public static SqlDataReader ExecuteSelectCommand(SqlCommand cmd)
{
if (cmd == null)
{
throw (new ArgumentNullException("cmd"));
}
SqlConnection con = new SqlConnection();
cmd.Connection = con;
con.Open();
SqlDataReader dr = cmd.ExecuteReader();
con.Close();
return dr ;
}
Class DatabuildLayer
SqlCommand com;
public DatabuildLayer()
{
com = new SqlCommand();
//
// TODO: Add constructor logic here
//
}
public SqlDataReader SelectCatalog(int catid)
{
DataAccessLayer.Setcommand(com, CommandType.Text, "select catname,catdescription,photo from category where catid=#catid" );
DataAccessLayer.addSQLparameter(com,16,SqlDbType.Int,catid,"#catid",ParameterDirection.Input);
return DataAccessLayer.ExecuteSelectCommand(com);;
}
and here my last code that retrieve my data to some textbox
in my Pageload :
protected void Page_Load(object sender, EventArgs e)
{
DatabuildLayer= new DatabuildLayer();
SqlDataReader dr ;
dr = obj.SelectCatalog(catselectddl.SelectedIndex);
if (dr.Read())
{
catnametxt.Text = dr["catname"].ToString();
catdestxt.Text = dr["catdescription"].ToString();
}
}
Is it possible that the query is returning nothing, and dr.Read() is returning false? Assuming the code actually executes (it is hard to tell from here) that is probably the only thing that would stop it working - either that or empty columns.
For what it is worth I think that your code needs to be tidied up a bit from a structural and conventions point of view. You should probably look through your code and consider the naming guidelines for the .NET framework. When others read your code they will want it formatted and consistent with this documentation. http://msdn.microsoft.com/en-us/library/xzf533w0(v=vs.71).aspx
Further, most people doing ASP.NET these days try to look for some way to inject external dependencies (such as databases) into their code using a framework like WebFormsMVP available at
http://webformsmvp.com/ in conjunction with an IoC container like autofac available at http://code.google.com/p/autofac/.
Using this approach you can push all external dependencies out of your application behind interfaces which would make it fairly trivial to plug in a different database engine.
Your current wrapper code is not doing anything particularly useful (just subsituting the existing methods or your own tht do the same thing), and it is not closing the connections correctly. It is... a bit of a mess.
If you aren't already massively familiar with the raw ADO.NET interfaces, then maybe consider something like "dapper" which will do all this for you, with a sane API:
short catid = 16;
using(var conn = GetOpenConnection()) {
var row = conn.Query(
"select catname,catdescription,photo from category where catid=#catid",
new { catid }).FirstOrDefault();
if(row != null) {
string name = row.catname, desc = row.catdescription;
// ...
}
}
Or if you have a class with CatName / CatDescription properties:
var obj = conn.Query<Catalogue>(
"select catname,catdescription,photo from category where catid=#catid",
new { catid }).FirstOrDefault();
from my experience, when you close a connection associated with a DataReader, nothing can be retrieved from the reader anymore.
//You closed the connection before returning the dr in the your method below:
public static SqlDataReader ExecuteSelectCommand(SqlCommand cmd)
{
if (cmd == null)
{
throw (new ArgumentNullException("cmd"));
}
SqlConnection con = new SqlConnection();
cmd.Connection = con;
con.Open();
SqlDataReader dr = cmd.ExecuteReader();
con.Close(); //here your connection was already closed
return dr ; //this dr is disconnected
}